Hitherto, attention has been paid to the utilization of EL displays using electroluminescence as luminescent elements in various display devices since the EL display devices have characteristics that the EL elements are capable of emitting light for itself, the capability of being watched and perceived is high, and they are superior in impact-resistance because of the perfect solid state thereof. Particularly, organic EL display devices using an organic compound as their luminescent material have been positively made practicable since the devices make it possible to lower voltage to be applied largely and can easily be made thin and small-sized so that the consumption power thereof can be made small.
Such an organic EL display device 100 is disclosed in JP-Laid/Open-Hei-10-289784 and JP-Laid/Open-Hei-11-185955. The outline of this device 100 is illustrated in FIG. 6. An organic EL element 130 is composed to sandwich an organic compound containing a luminescent layer 124 between a lower electrode (conductive layer) 122 deposited on a supporting substrate 121 and an upper electrode (hole injection electrode) 125, and a sealing member 126 for excluding the effect of moisture in the atmosphere is set above this organic EL element 130. Furthermore, a color filter layer 127 is disposed on the face, opposite to the organic EL element 130, of the sealing element 126. In the example of this organic EL element 130, a nonconductive layer 123 is arranged between the lower electrode 122 and the luminescent layer 124. A void (for example, gas such as nitrogen) layer 131 is present between the color filter layer 127 and the upper electrode 125.
Accordingly, by applying a given voltage between the upper and lower electrodes 122 and 125, EL emission passes from the side of the upper electrode 125, which is a transparent electrode, through the void layer 131, the color filter layer 127 and the sealing member 126 and then the EL emission can be taken out. In FIG. 6, an arrow represents the direction in which the EL emission is taken out.
As illustrated in FIG. 7, JP-Laid/Open-Hei-10-162958 discloses an organic EL display device 200 wherein color changing layers 201 and 202, a protective layer 203, a transparent electrode 204, an organic luminescent layer 205 and a rear electrode 220 are disposed below an insulating substrate (glass substrate) 210. The device 200 is composed so as to take out EL emission through the color changing layers 201 and 202 from the side of the transparent electrode 204.
Therefore, by applying a given voltage between the upper and the lower electrodes 204 and 220, EL emission passes from the side of the transparent electrode 204 through the protective layer 203, the color changing layers 201 and 202, and the insulating substrate 210 and then the EL emission can be taken out. In FIG. 7, an arrow represents the direction in which the EL emission is taken out.
In the case that light passes through an interface between two layers a and b made of constituent materials having different refractive indexes, the relationship between the reflectivity R of the interface (the reflectivity against light perpendicular to the interface) and the refractive indexes na and nb of the constituent materials of the two layers is represented by the following:R=(na−nb)2/(na+nb)2
As can be understood from this expression, therefore, the reflectivity R of the interface becomes larger as the difference between the refractive indexes na and nb of the constituent materials of the two layers becomes larger. As a result, the quantity of the light transmitting through the interface decreases.
For example, when light is radiated from an indium zinc oxide (IZO, refractive index: 2.1) and the light comes through a void layer (refractive index: 1.0) into a glass substrate (refractive index: 1.5), the light quantity incoming in the glass substrate is reduced to 84% of the light quantity (100%) outgoing from the IZO on the assumption that the light absorbance of the respective layers themselves are zero.
However, according to the organic EL display devices disclosed in JP-Laid/Open-Hei-10-289784 and 11-185955, as the transparent conductive material constituting the upper electrode, an indium tin oxide (ITO) having a refractive index of about 2 or the like material is used, and the refractive index of the void (gas such as nitrogen) layer between the upper electrode and the sealing member is 1. Therefore, the refractive index difference between the upper electrode and the void layer and the refractive index difference between the void layer and the sealing member become large. In general, the color changing layers are also made of a polymer material having a far larger refractive index value than that of the void layer. For this reason, the refractive index difference between the void layer and the color changing layers also becomes large. This results in a problem that EL emission reflects on the respective interfaces so that the quantity of the EL emission which can be taken out becomes remarkably small.
In the organic EL display device disclosed in JP-Laid/Open-Hei-10-162958, the relationship between the refractive indexes of the respective layers is not considered. Therefore, EL emission reflects on the respective interfaces. The number of the layers through the EL emission must transmit is large. These facts result in a problem that the quantity of the EL emission which can be taken out gets less.
JP-Laid/Open-Hei-7-272857 discloses an inorganic EL element formed on a supporting substrate and having a structure wherein, on its upper electrode side, a silicone oil (sealing agent) having a refractive index (s2) smaller than the refractive index (s1) of the upper electrode and a protective layer having a refractive index (s3) smaller than that of this silicone oil and larger than 1 are disposed and further EL emission is taken out from the side of the upper electrode.
However, when the silicone oil used in this inorganic EL element is used as a sealing agent of an organic EL element, the silicone oil may cause an organic luminescent medium in the organic EL element to be dissolved and invades into its layer interface to disturb its layer structure. Thus, the organic EL element may be deteriorated and the durability thereof may also be deteriorated.
In such an inorganic EL element, the luminescent brightness thereof is originally low. Thus, even if each of the refractive indexes of its upper electrode, sealing agent and protective layer is considered, it is practically difficult that the inorganic EL element exhibits performance equivalent to that of organic EL elements or is easily produced.
Thus, the inventors of the present invention have found out that by considering the relationship between the refractive indexes of a sealing member and a color changing medium and those of a transparent electrode and so on, the quantity of EL emission which can be taken out in an organic EL display device can be made large.
That is, an object is to provide an organic EL display device wherein even if a sealing member is set up and EL emission is taken out through the sealing member in a first invention, even if a color changing medium is set up and EL emission is taken out through the color changing medium in a second invention, or even if a color changing medium is arranged outside a supporting substrate and EL emission is taken out through the color changing medium in a third invention, reflection on each of interfaces is effectively suppressed so that the quantity of the EL emission which can be taken out is large.